Pulmonary embolism, an obstruction of the pulmonary artery or one of its branches by a blood clot or other foreign substance, is a major cause of in-hospital death. To prevent pulmonary embolism from occurring, patients are commonly treated with anticoagulants such as Heparin and with thrombolytic agents such as Streptokinase. In some situations, however, reliance on this approach to treatment may be inappropriate where, for example, the patient is Heparin sensitive or has a high risk of internal bleeding. Also, this approach is sometimes simply ineffective in preventing recurrent pulmonary emboli.
Mechanical interruption of the inferior vena cava presents an effective alternative when chemical treatment is inappropriate or ineffective. Most devices and methods currently available for mechanically interrupting the inferior vena cava, however, carry a number of significant disadvantages. For example, applying clips or sutures to create new, multiple and reduced orifices within the inferior vena cava to trap emboli and prevent them from migrating to the lungs is an operative procedure requiring anesthesia, creates increased post-operative complications, and may ultimately cause complete occlusion of the vena cava.
Improved treatment has been effected through the development of permanently placed filters. When a permanent filter has been implanted, though, the patient must remain on anticoagulant medicine for as long as the filter is left in place. Also, over the period of time that it is left in place, a permanent filter may eventually become dislodged or become clogged, and the filter must consequently be removed or replaced. While some permanent filters are designed to be percutaneously "retrievable", they often become embedded as their anchoring hooks or protrusions become endothelialized by the vessel wall and retrieval must be done surgically.
In many cases, where long term inferior vena cava filtration is not necessary or appropriate, a temporary filter that can be readily retrieved may provide a preferable alternative. One such device, disclosed in U.S. Pat. No. 5,549,626, includes inner and outer catheters and a dome-shaped, mesh-like, collapsible filter basket mounted to the distal end of the inner catheter. In use, the inner catheter and collapsed filter basket are telescopically advanced through the lumen of the outer catheter until the basket exits the distal end of the outer catheter where it expands to its domed shape to engage the vessel walls and trap emboli. A syringe may be applied to the inner catheter to aspirate the trapped particles from the filter. Aside from the mechanical complexity of using two coaxial catheters, the mesh-like material may obstruct the flow of blood in the blood vessel more than necessary, or may be tangled or torn during introduction, placement, expansion and retrieval. The smaller inner catheter and mesh-like basket also tend to interfere with and limit the access of other instruments to the blood vessel or the placement of a permanent filter. "A Retrievable Nitinol Vena Cava Filter: Experimental and Initial Clinical Results," by Nakagawa et al. in the Journal of Vascular and Interventional Radiology, May-June 1994, p. 507 describes a femorally deployed temporary filter made of Nitinol which forms a frustoconical shape that decreases in diameter as the filter extends distally away from its delivery sheath in the direction toward the heart. The distal end of the filter is formed into a larger diameter, stabilizing loop, which acts to center the terminating end of the filter within the blood vessel. Unfortunately, it is difficult to reach the interior of this filter to perform aspiration, and there is a risk that emboli loosened in the attempt to aspirate or in the process of removing the filter will pass back toward the heart. While a permanent filter may be placed above the temporary filter as a precaution against such an occurrence, doing so would appear to defeat much of the purpose of utilizing a temporary filter in the first place. While this filter is in place, it is also difficult to gain access to or through the filter area with other instrumentation when the need to do so arises.
What is needed is an improved filter for trapping emboli in a blood vessel where the filter is easy to deploy and retrieve and facilitates access to the vessel at or around the filtering site, for example, to aspirate or infuse.